This invention relates to a light measuring device, and more particularly to a light measuring device capable of measuring the power level of incident light.
In a light measuring device, such as an optical power meter, for measuring the power level of the incident light from an object to be measured, such as a light source or an optical transmission device externally connected to a photo-detecting system, the offset level of the photo-detecting system (e.g., an amplifier) must be compensated for.
Here, the offset level is defined as the output level of the photo-detecting system on which no light from the object to be measured is projected.
The accurate optical power of light from the measured object can be calculated by subtracting the offset level from the output level of the photo-detecting system on which the light from the measured object has been projected.
The subtraction process is called offset compensation.
In the case of conventional light measuring devices, before optical power is measured, offset compensation is made according to the following procedure.
First, a shade cover is put on the connector section of the light measuring device to which an object to be measured is connected.
Then, the output level of the light measuring device whose connector section has not been struck by light is measured and the measured level is determined to be the offset level.
Thereafter, the shade cover is removed and an object to be measured is connected to the connector section of the light measuring device. Then, the light from the object is allowed to strike the connector section.
Then, the previously determined offset level is subtracted from the level of the light sensed by the photo-detecting system of the light measuring device at that time. In this way, the resulting level is measured as the optical power level of light from the measured object.
The offset compensation is made periodically to reduce measurement errors.
With the conventional light measuring devices, however, light must be shaded to make the above-described offset compensation. This requires the laborious task of putting a shade cover on the connector section before measurement, measuring the offset level, removing the shade cover, connecting an object to be measured to the connector section of the light measuring device, and making the desired measurement.
Furthermore, the conventional light measuring devices prevent the desired measuring operation from being started immediately.
The causes of preventing the offset level from decreasing to zero with the conventional light measuring devices are roughly divided into the following two factors:
1. dark current in the photo-detecting element PA1 2. a direct-current amplifier connected to the photo-detecting element
The inventors of the specification have considered the effect of factor 1 on the measurement result.
FIG. 2 shows a characteristic of the output current, dark current, and current multiplication factor of an InGaAs APD (Avalanche Photodiode) versus reverse voltage.
In FIG. 2, when the optical power of the measured object is measured (with a reverse bias at a multiplication factor of about one), a dark current of about 10.sup.-10 A is generated.
The dark current is added to the received-light current caused by the light from the measured object. The APD outputs the resulting current.
FIG. 3 shows the calculated measurement error by the dark current in this condition corresponding to the optical power.
It is seen from FIG. 3 that if the optical power is in the range down to about -50 dBm, measurements can be made with an error of less than 0.05 dB.
Therefore, as shown in FIG. 3, when optical power is measured in the range down to -50 dBm, the necessity for compensating for the offset level due to factor 1 is small. As a result, only the offset level due to factor 2 has only to be compensated for.
One known light measuring device is an OTDR (an Optical Time Domain Reflectometer), which throws light pulse on a fiber to be measured, processes the reflected light (back scattering light or Fresnel reflected light) from the measured fiber as a result of the supply of the light pulse, and measures losses or defective points in the measured fiber.
FIG. 6 shows a general configuration of an OTDR of this type.
The OTDR comprises a timing generator section 21, a light pulse emitting section 22, a branch section 23, a photo-detecting section 24, a direct-current amplifying section 25, an A/D conversion section 26, a processing section 27, and a display section 28.
In the OTDR, on the basis of the control signal from the processing section 27, the timing generator section 21 outputs a signal to the light pulse emitting section 22 in a period corresponding to the length of a fiber 29 to be measured, that is, in the period T longer than time t required for the reflected light to come back the total length of the fiber 29 since the supply of the light pulse to the fiber 29.
Receiving the output, the light pulse generator section 22 generates a light pulse in each period T.
The light pulse generated in each period T by the light pulse generator section 22 is allowed to input the fiber 29 via the branch section 23.
The reflected light returning from the fiber 29 as a result of the supply of the light pulse is allowed to input the photo-detecting section 24 via the branch section 23, which converts the light into electricity.
The photoelectrically converted signal is converted by the A/D converter section 26 into a digital signal. The digital signal is inputted to the processing section 27.
The processing section 27 performs the process of logarithmically converting the inputted digital signal by sampling the data.
On the basis of the signal processing, a waveform is displayed on the display section 28.
With the OTDR of this type, a light pulse is thrown to the fiber 29 in each period T (e.g., at intervals of one msec).
Of an N number of (e.g., 5000) data items sampled during one period T, the average value of an M number of (e.g., 20) data items not containing the reflected light from the fiber 29 is determined to be the offset level.
Then, offset compensation is made by subtracting the offset level from each of the N number of data items sampled.
Recent OTDRs are required to have the function of measuring the power level of the incident light from the object, such as a light source or a light transmission device.
In the case of a configuration realizing such a function, the received-light power level of incident light is measured without using a light pulse from the light pulse emitting section 22.
For this reason, with the configuration realizing the above-described function, offset compensation for the photo-detecting system must be made in the OTDR as in generally used light measuring devices.